1. Field of the Invention
The present invention relates to a focus detection device having a predictive function, and in particular it relates to a focus detection device for a camera having a predictive function to determine a quantity relating to a future photographic field position or speed of movement of a photographic field when a photographic subject is moving close to or distant from a camera.
2. Description of the Related Art
Cameras are known which are capable of predicting a future photographic field position for a photographic subject based upon past focus detection results. Further, cameras are known which predict a future field position by utilizing focus detection results from among one or more intermittently output focus detection results, subject to the influence of any error contained in the previously determined focus detection results.
An example of a method for predicting the future field position of a moving photographic subject is disclosed in Japanese Laid-Open Patent Publication No. 63-148218 ("JP-A-63-148218"), which describes a method wherein a prediction is made using two focus detection results. The method utilized in JP-A-63-148218 is shown in FIGS. 27 and 28. As seen in FIG. 27, which plots the photographic field position Z on the ordinate and plots time t on the abscissa, a prediction is made using a focus detection result at t1, and a focus detection result at a time .DELTA.tL prior to t1. The broken-line arrows show the scatter in the error in the focus detection results. Based upon the scatter in the error of the focus detection results determined at t1 and at t1-.DELTA.tL, an estimated scatter of 40, as seen in the figure, is derived. FIG. 28 illustrates a situation where a prediction is made using two focus detection results at t1 and at a time .DELTA.tS prior to t1, where .DELTA.tS is smaller than .DELTA.tL. The broken-line arrows in FIG. 28 show the scatter in the error in the focus detection results. Based upon the scatter in the error of the focus detection results determined at t1 and t1-.DELTA.tS, an estimated scatter of 50, as seen in the figure, is derived. Although the degree of scatter depicted in FIGS. 27 and 28 is exaggerated for ease of understanding, it can be seen that according to the method disclosed in JP-A-63-148218 the prediction error increases as the time interval between focus detection results used for prediction becomes shorter. Thus, according to JP-A-63-148218, in order to increase the accuracy of prediction, focus detection results should be taken at suitable time intervals.
Prediction of the future field position using the above-described method is possible with good accuracy when the change of field movement speed is small. However, when the change in field movement speed is large, precise prediction is not possible. For example, FIGS. 29 and 30 graph the change of relative field position for a car moving from far to near opposite a photographic lens. As seen in FIGS. 29 and 30, field position Z is plotted on the ordinate and time t is plotted on the abscissa. The change in field movement speed at time t1 is small, and for the reasons already described with respect to FIGS. 27 and 28, at time t1, taking a larger time interval between focus detection results, as shown in FIG. 29, results in better accuracy of prediction. However, at time 12, in FIG. 29, when the change in field movement speed is large and the prediction is made using the result of a large time interval .DELTA.tL, an erroneous prediction is performed.
In order to solve the problem of erroneous prediction when there is a sudden change in the field movement speed, as seen, for example, at position t2 in FIG. 30, a smaller time interval .DELTA.tS between focus detection results is used. However, it is necessary to discriminate between the situation when the change in field movement speed is sudden, and the situation in which the change is gradual, such that the method of prediction may be modified based on the result of this discrimination.